Strange for loop behavior in Javascript function - javascript

I have this below loop in my function
I see it loop through if x is <10 but if it is greater than 10 it fails even though the condition of y < x satisfies
function insert(x,y) {
for (var shift = y; shift < x; shift++) {
//get into loop for insert (9,2) but not for insert(10,2)
}
}
This is the actual function, I am trying to visualize a insert before scenario using rapheal.js it works for insertBeforeTo(9,2) but when i try insertBeforeTo(10,2) it do not get into the loop.
function insertBeforeTo(whichElementI, insertBeforeI) {
var twhichElement = blocks[whichElementI];
blocks[whichElementI].animate({ x: blocks[insertBeforeI].attr('x') }, 1000, ">");
var shiftplusone = insertBeforeI;
for (var shift = insertBeforeI; shift < whichElementI; shift++) {
++shiftplusone;
blocks[shift].animate({ x: blocks[shiftplusone].attr('x') }, 1000, ">");// check value actually changes
}
}
Down voters: mind explaining?
Issue found: while debugging I see whichElementI and insertBeforeI values in ' '. So I assume It takes it as a string and as nnnnn and paxdiablo rightly pointed out, it takes as string instead of int so It worked for whichElementI = 9 and insertBeforeI = 2 and not whichElementI= 10 ,insertBeforeI=2.
So I used unary plus operator like +whichElementI, +insertBeforeI which fixes the issue.
Thanks

I'm guessing you are inadvertantly calling your function with string values rather than number values, that is with
insert("9", "2")
insert("10", "2")
...rather than
insert(9, 2)
insert(10, 2)
If you are getting your values from html element attributes, for example, they'll be strings.
With strings, "2" < "9" is true, but "2" < "10" is false - it does a lexicographic (dictionary) order comparison, not a numeric comparison.
If you want your function to work either way then you can change it to convert strings to numbers (I prefer to use the unary plus operator for that purpose):
function insert(x,y) {
x = +x;
y = +y;
for (var shift = y; shift < x; shift++) {
//get into loop for insert (9,2) but not for insert(10,2)
}
}

I don't know what the types of whichElementI and insertBeforeI are, but convert them to numeric by *1. That should help keep some of the arithmetic in check. Might also want to check if(isNaN(some_var)==false){...} as a sanity check.
function insertBeforeTo(whichElementI, insertBeforeI) {
var twhichElement = blocks[whichElementI];
blocks[whichElementI].animate({ x: blocks[insertBeforeI].attr('x') }, 1000, ">");
var shiftplusone = insertBeforeI*1;
for (var shift = insertBeforeI*1; shift < whichElementI*1; shift++) {
++shiftplusone;
blocks[shift].animate({ x: blocks[shiftplusone].attr('x') }, 1000, ">");// check x value actually changes
}
}

You should first check what data types you're passing in to the function.
For example, all of these produce output:
insert ( 10, 2)
insert ( 9, 2)
insert ( '9', '2')
but the following does not:
insert ( '10', '2')
That's because the string '10' is actually less than the string '2'. By that, I mean that a string is not treated numerically when sorting or comparing, rather each character is compared individually, leading to things like the first twelve numbers being sequenced as:
1
10
11
12
2
3
4
5
6
7
8
9
You can see this in action with the following code (at one of the many on-line JS runners):
function insert(x,y) {
for (var shift = y; shift < x; shift++) {
//get into loop for insert (9,2) but not for insert(10,2)
document.write('x');
}
}
document.write('<br>10,2 as string: '); insert ('10','2');
document.write('<br>9,2 as string: '); insert ( '9','2');
document.write('<br>9,2 as num: '); insert ( 9, 2);
document.write('<br>10,2 as num: '); insert ( 10, 2);
which outputs:
10,2 as string:
9,2 as string: xxxxxxx
9,2 as num: xxxxxxx
10,2 as num: xxxxxxxx
If you have a string that you want treated as a number, parseInt() is one way to go, something like:
var str42 = '42';
var num42 = parseInt (str42, 10);

Related

How to generate random text when button is clicked? [duplicate]

I want a 5 character string composed of characters picked randomly from the set [a-zA-Z0-9].
What's the best way to do this with JavaScript?
I think this will work for you:
function makeid(length) {
let result = '';
const characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
const charactersLength = characters.length;
let counter = 0;
while (counter < length) {
result += characters.charAt(Math.floor(Math.random() * charactersLength));
counter += 1;
}
return result;
}
console.log(makeid(5));
//Can change 7 to 2 for longer results.
let r = (Math.random() + 1).toString(36).substring(7);
console.log("random", r);
Note: The above algorithm has the following weaknesses:
It will generate anywhere between 0 and 6 characters due to the fact that trailing zeros get removed when stringifying floating points.
It depends deeply on the algorithm used to stringify floating point numbers, which is horrifically complex. (See the paper "How to Print Floating-Point Numbers Accurately".)
Math.random() may produce predictable ("random-looking" but not really random) output depending on the implementation. The resulting string is not suitable when you need to guarantee uniqueness or unpredictability.
Even if it produced 6 uniformly random, unpredictable characters, you can expect to see a duplicate after generating only about 50,000 strings, due to the birthday paradox. (sqrt(36^6) = 46656)
Math.random is bad for this kind of thing
server side
Use node crypto module -
var crypto = require("crypto");
var id = crypto.randomBytes(20).toString('hex');
// "bb5dc8842ca31d4603d6aa11448d1654"
The resulting string will be twice as long as the random bytes you generate; each byte encoded to hex is 2 characters. 20 bytes will be 40 characters of hex.
client side
Use the browser's crypto module, crypto.getRandomValues -
The crypto.getRandomValues() method lets you get cryptographically strong random values. The array given as the parameter is filled with random numbers (random in its cryptographic meaning).
// dec2hex :: Integer -> String
// i.e. 0-255 -> '00'-'ff'
function dec2hex (dec) {
return dec.toString(16).padStart(2, "0")
}
// generateId :: Integer -> String
function generateId (len) {
var arr = new Uint8Array((len || 40) / 2)
window.crypto.getRandomValues(arr)
return Array.from(arr, dec2hex).join('')
}
console.log(generateId())
// "82defcf324571e70b0521d79cce2bf3fffccd69"
console.log(generateId(20))
// "c1a050a4cd1556948d41"
A step-by-step console example -
> var arr = new Uint8Array(4) # make array of 4 bytes (values 0-255)
> arr
Uint8Array(4) [ 0, 0, 0, 0 ]
> window.crypto
Crypto { subtle: SubtleCrypto }
> window.crypto.getRandomValues()
TypeError: Crypto.getRandomValues requires at least 1 argument, but only 0 were passed
> window.crypto.getRandomValues(arr)
Uint8Array(4) [ 235, 229, 94, 228 ]
For IE11 support you can use -
(window.crypto || window.msCrypto).getRandomValues(arr)
For browser coverage see https://caniuse.com/#feat=getrandomvalues
client side (old browsers)
If you must support old browsers, consider something like uuid -
const uuid = require("uuid");
const id = uuid.v4();
// "110ec58a-a0f2-4ac4-8393-c866d813b8d1"
Short, easy and reliable
Returns exactly 5 random characters, as opposed to some of the top rated answers found here.
Math.random().toString(36).slice(2, 7);
Here's an improvement on doubletap's excellent answer. The original has two drawbacks which are addressed here:
First, as others have mentioned, it has a small probability of producing short strings or even an empty string (if the random number is 0), which may break your application. Here is a solution:
(Math.random().toString(36)+'00000000000000000').slice(2, N+2)
Second, both the original and the solution above limit the string size N to 16 characters. The following will return a string of size N for any N (but note that using N > 16 will not increase the randomness or decrease the probability of collisions):
Array(N+1).join((Math.random().toString(36)+'00000000000000000').slice(2, 18)).slice(0, N)
Explanation:
Pick a random number in the range [0,1), i.e. between 0 (inclusive) and 1 (exclusive).
Convert the number to a base-36 string, i.e. using characters 0-9 and a-z.
Pad with zeros (solves the first issue).
Slice off the leading '0.' prefix and extra padding zeros.
Repeat the string enough times to have at least N characters in it (by Joining empty strings with the shorter random string used as the delimiter).
Slice exactly N characters from the string.
Further thoughts:
This solution does not use uppercase letters, but in almost all cases (no pun intended) it does not matter.
The maximum string length at N = 16 in the original answer is measured in Chrome. In Firefox it's N = 11. But as explained, the second solution is about supporting any requested string length, not about adding randomness, so it doesn't make much of a difference.
All returned strings have an equal probability of being returned, at least as far as the results returned by Math.random() are evenly distributed (this is not cryptographic-strength randomness, in any case).
Not all possible strings of size N may be returned. In the second solution this is obvious (since the smaller string is simply being duplicated), but also in the original answer this is true since in the conversion to base-36 the last few bits may not be part of the original random bits. Specifically, if you look at the result of Math.random().toString(36), you'll notice the last character is not evenly distributed. Again, in almost all cases it does not matter, but we slice the final string from the beginning rather than the end of the random string so that short strings (e.g. N=1) aren't affected.
Update:
Here are a couple other functional-style one-liners I came up with. They differ from the solution above in that:
They use an explicit arbitrary alphabet (more generic, and suitable to the original question which asked for both uppercase and lowercase letters).
All strings of length N have an equal probability of being returned (i.e. strings contain no repetitions).
They are based on a map function, rather than the toString(36) trick, which makes them more straightforward and easy to understand.
So, say your alphabet of choice is
var s = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
Then these two are equivalent to each other, so you can pick whichever is more intuitive to you:
Array(N).join().split(',').map(function() { return s.charAt(Math.floor(Math.random() * s.length)); }).join('');
and
Array.apply(null, Array(N)).map(function() { return s.charAt(Math.floor(Math.random() * s.length)); }).join('');
Edit:
I seems like qubyte and Martijn de Milliano came up with solutions similar to the latter (kudos!), which I somehow missed. Since they don't look as short at a glance, I'll leave it here anyway in case someone really wants a one-liner :-)
Also, replaced 'new Array' with 'Array' in all solutions to shave off a few more bytes.
The most compact solution, because slice is shorter than substring. Subtracting from the end of the string allows to avoid floating point symbol generated by the random function:
Math.random().toString(36).slice(-5);
or even
(+new Date).toString(36).slice(-5);
Update: Added one more approach using btoa method:
btoa(Math.random()).slice(0, 5);
btoa(+new Date).slice(-7, -2);
btoa(+new Date).substr(-7, 5);
// Using Math.random and Base 36:
console.log(Math.random().toString(36).slice(-5));
// Using new Date and Base 36:
console.log((+new Date).toString(36).slice(-5));
// Using Math.random and Base 64 (btoa):
console.log(btoa(Math.random()).slice(0, 5));
// Using new Date and Base 64 (btoa):
console.log(btoa(+new Date).slice(-7, -2));
console.log(btoa(+new Date).substr(-7, 5));
A newer version with es6 spread operator:
[...Array(30)].map(() => Math.random().toString(36)[2]).join('')
The 30 is an arbitrary number, you can pick any token length you want
The 36 is the maximum radix number you can pass to numeric.toString(), which means all numbers and a-z lowercase letters
The 2 is used to pick the 3rd index from the random string which looks like this: "0.mfbiohx64i", we could take any index after 0.
Something like this should work
function randomString(len, charSet) {
charSet = charSet || 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
var randomString = '';
for (var i = 0; i < len; i++) {
var randomPoz = Math.floor(Math.random() * charSet.length);
randomString += charSet.substring(randomPoz,randomPoz+1);
}
return randomString;
}
Call with default charset [a-zA-Z0-9] or send in your own:
var randomValue = randomString(5);
var randomValue = randomString(5, 'PICKCHARSFROMTHISSET');
function randomstring(L) {
var s = '';
var randomchar = function() {
var n = Math.floor(Math.random() * 62);
if (n < 10) return n; //1-10
if (n < 36) return String.fromCharCode(n + 55); //A-Z
return String.fromCharCode(n + 61); //a-z
}
while (s.length < L) s += randomchar();
return s;
}
console.log(randomstring(5));
Random String Generator (Alpha-Numeric | Alpha | Numeric)
/**
* Pseudo-random string generator
* http://stackoverflow.com/a/27872144/383904
* Default: return a random alpha-numeric string
*
* #param {Integer} len Desired length
* #param {String} an Optional (alphanumeric), "a" (alpha), "n" (numeric)
* #return {String}
*/
function randomString(len, an) {
an = an && an.toLowerCase();
var str = "",
i = 0,
min = an == "a" ? 10 : 0,
max = an == "n" ? 10 : 62;
for (; i++ < len;) {
var r = Math.random() * (max - min) + min << 0;
str += String.fromCharCode(r += r > 9 ? r < 36 ? 55 : 61 : 48);
}
return str;
}
console.log(randomString(10)); // i.e: "4Z8iNQag9v"
console.log(randomString(10, "a")); // i.e: "aUkZuHNcWw"
console.log(randomString(10, "n")); // i.e: "9055739230"
While the above uses additional checks for the desired A/N, A, N output,
let's break it down the to the essentials (Alpha-Numeric only) for a better understanding:
Create a function that accepts an argument (desired length of the random String result)
Create an empty string like var str = ""; to concatenate random characters
Inside a loop create a rand index number from 0 to 61 (0..9+A..Z+a..z = 62)
Create a conditional logic to Adjust/fix rand (since it's 0..61) incrementing it by some number (see examples below) to get back the right CharCode number and the related Character.
Inside the loop concatenate to str a String.fromCharCode( incremented rand )
Let's picture the ASCII Character table ranges:
_____0....9______A..........Z______a..........z___________ Character
| 10 | | 26 | | 26 | Tot = 62 characters
48....57 65..........90 97..........122 CharCode ranges
Math.floor( Math.random * 62 ) gives a range from 0..61 (what we need).
Let's fix the random to get the correct charCode ranges:
| rand | charCode | (0..61)rand += fix = charCode ranges |
------+----------+----------+--------------------------------+-----------------+
0..9 | 0..9 | 48..57 | rand += 48 = 48..57 |
A..Z | 10..35 | 65..90 | rand += 55 /* 90-35 = 55 */ = 65..90 |
a..z | 36..61 | 97..122 | rand += 61 /* 122-61 = 61 */ = 97..122 |
The conditional operation logic from the table above:
rand += rand>9 ? ( rand<36 ? 55 : 61 ) : 48 ;
// rand += true ? ( true ? 55 else 61 ) else 48 ;
From the explanation above, here's the resulting alpha-numeric snippet:
function randomString(len) {
var str = ""; // String result
for (var i = 0; i < len; i++) { // Loop `len` times
var rand = Math.floor(Math.random() * 62); // random: 0..61
var charCode = rand += rand > 9 ? (rand < 36 ? 55 : 61) : 48; // Get correct charCode
str += String.fromCharCode(charCode); // add Character to str
}
return str; // After all loops are done, return the concatenated string
}
console.log(randomString(10)); // i.e: "7GL9F0ne6t"
Or if you will:
const randomString = (n, r='') => {
while (n--) r += String.fromCharCode((r=Math.random()*62|0, r+=r>9?(r<36?55:61):48));
return r;
};
console.log(randomString(10))
To meet requirement [a-zA-Z0-9] and length of 5 characters, use
For Browser:
btoa(Math.random().toString()).substring(10,15);
For NodeJS:
Buffer.from(Math.random().toString()).toString("base64").substring(10,15);
Lowercase letters, uppercase letters, and numbers will occur.
(it's typescript compatible)
The simplest way is:
(new Date%9e6).toString(36)
This generate random strings of 5 characters based on the current time. Example output is 4mtxj or 4mv90 or 4mwp1
The problem with this is that if you call it two times on the same second, it will generate the same string.
The safer way is:
(0|Math.random()*9e6).toString(36)
This will generate a random string of 4 or 5 characters, always diferent. Example output is like 30jzm or 1r591 or 4su1a
In both ways the first part generate a random number. The .toString(36) part cast the number to a base36 (alphadecimal) representation of it.
Here are some easy one liners. Change new Array(5) to set the length.
Including 0-9a-z
new Array(5).join().replace(/(.|$)/g, function(){return ((Math.random()*36)|0).toString(36);})
Including 0-9a-zA-Z
new Array(5).join().replace(/(.|$)/g, function(){return ((Math.random()*36)|0).toString(36)[Math.random()<.5?"toString":"toUpperCase"]();});
Codegolfed for ES6 (0-9a-z)
Array(5).fill().map(n=>(Math.random()*36|0).toString(36)).join('')
I know everyone has got it right already, but i felt like having a go at this one in the most lightweight way possible(light on code, not CPU):
function rand(length, current) {
current = current ? current : '';
return length ? rand(--length, "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz".charAt(Math.floor(Math.random() * 60)) + current) : current;
}
console.log(rand(5));
It takes a bit of time to wrap your head around, but I think it really shows how awesome javascript's syntax is.
Generate a secure random alphanumeric Base-62 string:
function generateUID(length)
{
return window.btoa(String.fromCharCode(...window.crypto.getRandomValues(new Uint8Array(length * 2)))).replace(/[+/]/g, "").substring(0, length);
}
console.log(generateUID(22)); // "yFg3Upv2cE9cKOXd7hHwWp"
console.log(generateUID(5)); // "YQGzP"
There is no best way to do this. You can do it any way you prefer, as long as the result suits your requirements. To illustrate, I've created many different examples, all which should provide the same end-result
Most other answers on this page ignore the upper-case character requirement.
Here is my fastest solution and most readable. It basically does the same as the accepted solution, except it is a bit faster.
function readableRandomStringMaker(length) {
for (var s=''; s.length < length; s += 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'.charAt(Math.random()*62|0));
return s;
}
console.log(readableRandomStringMaker(length));
// e3cbN
Here is a compact, recursive version which is much less readable:
const compactRandomStringMaker = (length) => length-- && "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0) + (compactRandomStringMaker(length)||"");
console.log(compactRandomStringMaker(5));
// DVudj
A more compact one-liner:
Array(5).fill().map(()=>"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62)).join("")
// 12oEZ
A variation of the above:
" ".replaceAll(" ",()=>"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62))
The most compact one-liner, but inefficient and unreadable - it adds random characters and removes illegal characters until length is l:
((l,f=(p='')=>p.length<l?f(p+String.fromCharCode(Math.random()*123).replace(/[^a-z0-9]/i,'')):p)=>f())(5)
A cryptographically secure version, which is wasting entropy for compactness, and is a waste regardless because the generated string is so short:
[...crypto.getRandomValues(new Uint8Array(999))].map((c)=>String.fromCharCode(c).replace(/[^a-z0-9]/i,'')).join("").substr(0,5)
// 8fzPq
Or, without the length-argument it is even shorter:
((f=(p='')=>p.length<5?f(p+String.fromCharCode(Math.random()*123).replace(/[^a-z0-9]/i,'')):p)=>f())()
// EV6c9
Then a bit more challenging - using a nameless recursive arrow function:
((l,s=((l)=>l--&&"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0)+(s(l)||""))) => s(l))(5);
// qzal4
This is a "magic" variable which provides a random character every time you access it:
const c = new class { [Symbol.toPrimitive]() { return "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0) } };
console.log(c+c+c+c+c);
// AgMnz
A simpler variant of the above:
const c=()=>"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0);
c()+c()+c()+c()+c();
// 6Qadw
In case anyone is interested in a one-liner (although not formatted as such for your convenience) that allocates the memory at once (but note that for small strings it really does not matter) here is how to do it:
Array.apply(0, Array(5)).map(function() {
return (function(charset){
return charset.charAt(Math.floor(Math.random() * charset.length))
}('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'));
}).join('')
You can replace 5 by the length of the string you want. Thanks to #AriyaHidayat in this post for the solution to the map function not working on the sparse array created by Array(5).
If you are using Lodash or Underscore, then it so simple:
var randomVal = _.sample('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', 5).join('');
const c = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
const s = [...Array(5)].map(_ => c[~~(Math.random()*c.length)]).join('')
Here's the method I created.
It will create a string containing both uppercase and lowercase characters.
In addition I've included the function that will created an alphanumeric string too.
Working examples:
http://jsfiddle.net/greatbigmassive/vhsxs/ (alpha only)
http://jsfiddle.net/greatbigmassive/PJwg8/ (alphanumeric)
function randString(x){
var s = "";
while(s.length<x&&x>0){
var r = Math.random();
s+= String.fromCharCode(Math.floor(r*26) + (r>0.5?97:65));
}
return s;
}
Upgrade July 2015
This does the same thing but makes more sense and includes all letters.
var s = "";
while(s.length<x&&x>0){
v = Math.random()<0.5?32:0;
s += String.fromCharCode(Math.round(Math.random()*((122-v)-(97-v))+(97-v)));
}
One liner:
Array(15).fill(null).map(() => Math.random().toString(36).substr(2)).join('')
// Outputs: 0h61cbpw96y83qtnunwme5lxk1i70a6o5r5lckfcyh1dl9fffydcfxddd69ada9tu9jvqdx864xj1ul3wtfztmh2oz2vs3mv6ej0fe58ho1cftkjcuyl2lfkmxlwua83ibotxqc4guyuvrvtf60naob26t6swzpil
Improved #Andrew's answer above :
Array.from({ length : 1 }, () => Math.random().toString(36)[2]).join('');
Base 36 conversion of the random number is inconsistent, so selecting a single indice fixes that. You can change the length for a string with the exact length desired.
Assuming you use underscorejs it's possible to elegantly generate random string in just two lines:
var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
var random = _.sample(possible, 5).join('');
function randomString (strLength, charSet) {
var result = [];
strLength = strLength || 5;
charSet = charSet || 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
while (strLength--) { // (note, fixed typo)
result.push(charSet.charAt(Math.floor(Math.random() * charSet.length)));
}
return result.join('');
}
This is as clean as it will get. It is fast too, http://jsperf.com/ay-random-string.
Fast and improved algorithm. Does not guarantee uniform (see comments).
function getRandomId(length) {
if (!length) {
return '';
}
const possible =
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
let array;
if ('Uint8Array' in self && 'crypto' in self && length <= 65536) {
array = new Uint8Array(length);
self.crypto.getRandomValues(array);
} else {
array = new Array(length);
for (let i = 0; i < length; i++) {
array[i] = Math.floor(Math.random() * 62);
}
}
let result = '';
for (let i = 0; i < length; i++) {
result += possible.charAt(array[i] % 62);
}
return result;
}
How about this compact little trick?
var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
var stringLength = 5;
function pickRandom() {
return possible[Math.floor(Math.random() * possible.length)];
}
var randomString = Array.apply(null, Array(stringLength)).map(pickRandom).join('');
You need the Array.apply there to trick the empty array into being an array of undefineds.
If you're coding for ES2015, then building the array is a little simpler:
var randomString = Array.from({ length: stringLength }, pickRandom).join('');
You can loop through an array of items and recursively add them to a string variable, for instance if you wanted a random DNA sequence:
function randomDNA(len) {
len = len || 100
var nuc = new Array("A", "T", "C", "G")
var i = 0
var n = 0
s = ''
while (i <= len - 1) {
n = Math.floor(Math.random() * 4)
s += nuc[n]
i++
}
return s
}
console.log(randomDNA(5));
Case Insensitive Alphanumeric Chars:
function randStr(len) {
let s = '';
while (s.length < len) s += Math.random().toString(36).substr(2, len - s.length);
return s;
}
// usage
console.log(randStr(50));
The benefit of this function is that you can get different length random string and it ensures the length of the string.
Case Sensitive All Chars:
function randStr(len) {
let s = '';
while (len--) s += String.fromCodePoint(Math.floor(Math.random() * (126 - 33) + 33));
return s;
}
// usage
console.log(randStr(50));
Custom Chars
function randStr(len, chars='abc123') {
let s = '';
while (len--) s += chars[Math.floor(Math.random() * chars.length)];
return s;
}
// usage
console.log(randStr(50));
console.log(randStr(50, 'abc'));
console.log(randStr(50, 'aab')); // more a than b
The problem with responses to "I need random strings" questions (in whatever language) is practically every solution uses a flawed primary specification of string length. The questions themselves rarely reveal why the random strings are needed, but I would challenge you rarely need random strings of length, say 8. What you invariably need is some number of unique strings, for example, to use as identifiers for some purpose.
There are two leading ways to get strictly unique strings: deterministically (which is not random) and store/compare (which is onerous). What do we do? We give up the ghost. We go with probabilistic uniqueness instead. That is, we accept that there is some (however small) risk that our strings won't be unique. This is where understanding collision probability and entropy are helpful.
So I'll rephrase the invariable need as needing some number of strings with a small risk of repeat. As a concrete example, let's say you want to generate a potential of 5 million IDs. You don't want to store and compare each new string, and you want them to be random, so you accept some risk of repeat. As example, let's say a risk of less than 1 in a trillion chance of repeat. So what length of string do you need? Well, that question is underspecified as it depends on the characters used. But more importantly, it's misguided. What you need is a specification of the entropy of the strings, not their length. Entropy can be directly related to the probability of a repeat in some number of strings. String length can't.
And this is where a library like EntropyString can help. To generate random IDs that have less than 1 in a trillion chance of repeat in 5 million strings using entropy-string:
import {Random, Entropy} from 'entropy-string'
const random = new Random()
const bits = Entropy.bits(5e6, 1e12)
const string = random.string(bits)
"44hTNghjNHGGRHqH9"
entropy-string uses a character set with 32 characters by default. There are other predefined characters sets, and you can specify your own characters as well. For example, generating IDs with the same entropy as above but using hex characters:
import {Random, Entropy, charSet16} from './entropy-string'
const random = new Random(charSet16)
const bits = Entropy.bits(5e6, 1e12)
const string = random.string(bits)
"27b33372ade513715481f"
Note the difference in string length due to the difference in total number of characters in the character set used. The risk of repeat in the specified number of potential strings is the same. The string lengths are not. And best of all, the risk of repeat and the potential number of strings is explicit. No more guessing with string length.
One-liner using map that gives you full control on the length and characters.
const rnd = (len, chars='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789') => [...Array(len)].map(() => chars.charAt(Math.floor(Math.random() * chars.length))).join('')
console.log(rnd(12))

JavaScript Roman Numeral Converter

I am not able to understand this solution properly. I understood the Array declaration part but I am not sure what's going on in the while loop.
function roman(num) {
var decimalValue = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
var romanNumeral = ['M', 'CM', 'D', 'CD', 'C', 'XC', 'L', 'XL', 'X', 'IX', 'V', 'IV', 'I'];
var romanized = '';
for (var index = 0; index < decimalValue.length; index++) {
while (decimalValue[index] <= num) {
romanized += romanNumeral[index];
num -= decimalValue[index];
}
}
return romanized;
}
Next time you're stuck on a loop-based problem like this, I would suggest learning about the debugger; command and breakpoints.
Let's use a specific number as an example, say... 2,652. Here's what will happen:
Starting with 1,000 (the first number in decimalValue), check if 2,562 > 1,000.
It is! So, we know that the Roman numeral for 2,652 has at least one M in it. We add M to our output Roman numeral.
We've "accounted for" 1,000 of our number as a numeral, so we remove 1,000 from the number. We now have 1,562.
We jump back up to checking if num > 1,000. It still is. So, we add another M and subtract another 1,000. Now we have 562 of our number "unaccounted for."
This time, when we jump up to the start of our while() loop to test if num > 1,000, we find that it isn't the case! So the loop does not run this time.
This process repeats for all the numbers in decimalValue; all the numbers that have Roman numeral equivalents.
So, we check 900/CM, and find that this number (which is now down to 562) can not be represented as a sum including 900.
Next, we check 500, and find that it can! We add the Roman numeral for 500 to our current romanized string and carry on. We now have MMD and our number is down to 62 "unaccounted for" digits/units/whatever we're counting.
The next number to catch our while() loop is 50, since 62 > 50. We add the L for 50 and bring our number down to 12. Then again on 10, and we add an X. Finally, we match on the last item in decimalValue, 1, twice, and add two Is.
Our final string for this number, 2,652, is MMDXII.
The While loop does a comparison starting from index zero to the last and compares if the value of num passed in is less that the value at index of decimal value array if it is, It will then it appends(concatenate) the ruman numeral at that particular index to the randomize and subtract the equivalent number in decimal(mutate the num variable) from the num which was sent in.
It then checks if num is still greater than the value at that particular index indicating decimal.
taking a walk through with 3002 as example.
First check if index 0 which has 1000 is less than 3002 true
Set randomize to ruman numeral at position index. In this case we have 'M' then subtract decimal at position index from num(3002) we now have num = 2002.
The while loop iterate again and check is 1000 less than 2002? yes it is so it will execute the body of the while loop again.
Append(Concatenate the value at position index(which has not changed yet) to the randomize variable this case index is still zero so we appending 'M' now randomize is 'MM'. Subtract the decimal at position index(0) from num(2002) we now have num = 1002.
Iterate the while loop and check if decimal value at position index(0) of the decimalValue array is less than num(1002) which is true in this case. execute the loop as before.
Append(Concatenate the value at position index(which has not changed yet) to the randomize variable this case index is still zero so we appending 'M' now randomize is 'MMM'. Subtract the decimal at position index(0) from num(1002) we now have num = 2.
Iterate the while loop and check if decimal value at position index(0) of the decimalValue array is less than num(2) which is false in this case. Stop execution of the loop and increment the value of index in the for loop and do the checks again. till you reach the end.
decimalValue[i] represents the same value as romanNumeral[i] for any i. This represents the same data as if the author used a object {4:"IV", 5:"V", ...} except objects to not preserve order. Since the author wants to check larger numbers before smaller numbers, they use this to preserve order and also associate the decimal values with the Roman Numerals.
The loop is confusing because javascript uses + to represent both numerical addition, 2+4==6, and string concatenation, "a"+"b"=="ab".
romanized += romanNumeral[index] means append the string contained in romanNumberal[index] to the end of the string romanized.
num -= decimalValue[index]; means decrease num by the number contained in decimalValue[index]
//start at the big values that have roman number codes, then try smaller values
for (var index = 0; index < decimalValue.length; index++) {
//if the value we are testing is less or equal to num
// (actually loop but for reasons explained below)
while (decimalValue[index] <= num) {
//append the string in romanNumberal to
//the end of the string in the variable romanized
romanized += romanNumeral[index];
//from the number num, substract the
//value we just added to romanized
num -= decimalValue[index];
//what about a case like 3, which needs
//repeated letters in that case "III".
//That is why this is a while statement and not an if.
//the while loop keeps processing
//those cases until we have enough repeated letters.
}
}
When we need more than one of the same the same letter, then decimalvalue[index] will still be less than or equal to num.
This is because when we reach decimalvalue[index], we know that num < decimalvalue[index-1] (except for the start), because the while loop prevents index from increasing until that is true.
Often the loop only runs once. For example, going from decimalvalue[index] == 10 to decimalvalue[index] == 9 loop only runs once at most because num < 10 and 10-9==1 which is less than 9. So then, the while loop acts as just an if statement.
The loop only loops for values where duplicate letters are valid in roman numerals, because that is were the drop in value between decimalvalue[i-1] and decimal[i] is great enough. More precisely those are the only times when decimalvalue[i-1]/decimalvalue[i] >= 2 (that is 2 or more of a value in decimalvalue[i] fits in the previous decimalvalue)
if you know the division algorithm from math this is kinda like that.
Found this codepen on google. The code is pretty straightforward, I hope it will help.
Roman Numeral Converter in Javascript
function convert(num){
num = parseInt(num);
var result = '',
ref = ['M','CM','D','CD','C','XC','L','XL','X','IX','V','IV','I'],
xis = [1000,900,500,400,100,90,50,40,10,9,5,4,1];
if (num >= 4000) {
num += ''; // need to convert to string for .substring()
result = '<span style="border-top: 1px solid; margin-top: 2px; display: inline-block; padding-top: 0px;">'+convert(num.substring(0,num.length-3))+'</span>';
num = num.substring(num.length-3);
}
for (x = 0; x < ref.length; x++){
while(num >= xis[x]){
result += ref[x];
num -= xis[x];
}
}
return result;
}
$('input').on('keyup keydown change',function(e){
var $this = $(this),
val = $this.val();
if (val.length == 0) return $('#result').html('');
if (isNaN(val)) return $('#result').html('Invalid input');
if (e.type == 'keydown'){
if (e.keyCode === 38) $this.val(++val);
if (e.keyCode === 40) $this.val(--val);
}
if (val < 1) return $('#result').html('Number is too small');
$('#result').html(convert(val));
})

Creating a string of digits in javascript each within a limit

How can I use javascript to randomly create a 20 digit string of numbers, each of the digits ranging only between 1 and 5?
An example would be: 52431425331425141521
As well as the logical algorithm I gave in my comment above, you could just use this one-liner:
var result = Math.floor(Math.random()*95367431640625).toString(5)
.split("").map(function(n) {return +n+1;}).join("");
Essentially, pick a random integer between 0 and 520-1, convert it to base 5, then increment all the digits by one, so they're all between 1 and 5 ^_^
EDIT: Just realised this won't handle low numbers too well. Try this:
var result = (
new Array(20).join("0")
+
Math.floor(Math.random()*95367431640625).toString(5)
).slice(-20).split("").map(function(n) {return +n+1;}).join(""));
This does basically the same, except it prepends 19 zeroes to the front of your number, then slices off the last 20 characters. This will allow it to handle leading zeroes correctly to give a 20-digit number in all cases.
You can use this:
function random_string()
{
var text = "";
var string = "12345";
for( var i=0; i < 20; i++ )
text += string.charAt(Math.floor(Math.random() * string.length));
return text;
}
random_string();

Math.random() returns value greater than one?

While playing around with random numbers in JavaScript I discovered a surprising bug, presumably in the V8 JavaScript engine in Google Chrome. Consider:
// Generate a random number [1,5].
var rand5 = function() {
return parseInt(Math.random() * 5) + 1;
};
// Return a sample distribution over MAX times.
var testRand5 = function(dist, max) {
if (!dist) { dist = {}; }
if (!max) { max = 5000000; }
for (var i=0; i<max; i++) {
var r = rand5();
dist[r] = (dist[r] || 0) + 1;
}
return dist;
};
Now when I run testRand5() I get the following results (of course, differing slightly with each run, you might need to set "max" to a higher value to reveal the bug):
var d = testRand5();
d = {
1: 1002797,
2: 998803,
3: 999541,
4: 1000851,
5: 998007,
10: 1 // XXX: Math.random() returned 4.5?!
}
Interestingly, I see comparable results in node.js, leading me to believe it's not specific to Chrome. Sometimes there are different or multiple mystery values (7, 9, etc).
Can anyone explain why I might be getting the results I see? I'm guessing it has something to do with using parseInt (instead of Math.floor()) but I'm still not sure why it could happen.
The edge case occurs when you happen to generate a very small number, expressed with an exponent, like this for example 9.546056389808655e-8.
Combined with parseInt, which interprets the argument as a string, hell breaks loose. And as suggested before me, it can be solved using Math.floor.
Try it yourself with this piece of code:
var test = 9.546056389808655e-8;
console.log(test); // prints 9.546056389808655e-8
console.log(parseInt(test)); // prints 9 - oh noes!
console.log(Math.floor(test)) // prints 0 - this is better
Of course, it's a parseInt() gotcha. It converts its argument to a string first, and that can force scientific notation which will cause parseInt to do something like this:
var x = 0.000000004;
(x).toString(); // => "4e-9"
parseInt(x); // => 4
Silly me...
I would suggest changing your random number function to this:
var rand5 = function() {
return(Math.floor(Math.random() * 5) + 1);
};
This will reliably generate an integer value between 1 and 5 inclusive.
You can see your test function in action here: http://jsfiddle.net/jfriend00/FCzjF/.
In this case, parseInt isn't the best choice because it's going to convert your float to a string which can be a number of different formats (including scientific notation) and then try to parse an integer out of it. Much better to just operate on the float directly with Math.floor().

Generate random string/characters in JavaScript

I want a 5 character string composed of characters picked randomly from the set [a-zA-Z0-9].
What's the best way to do this with JavaScript?
I think this will work for you:
function makeid(length) {
let result = '';
const characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
const charactersLength = characters.length;
let counter = 0;
while (counter < length) {
result += characters.charAt(Math.floor(Math.random() * charactersLength));
counter += 1;
}
return result;
}
console.log(makeid(5));
//Can change 7 to 2 for longer results.
let r = (Math.random() + 1).toString(36).substring(7);
console.log("random", r);
Note: The above algorithm has the following weaknesses:
It will generate anywhere between 0 and 6 characters due to the fact that trailing zeros get removed when stringifying floating points.
It depends deeply on the algorithm used to stringify floating point numbers, which is horrifically complex. (See the paper "How to Print Floating-Point Numbers Accurately".)
Math.random() may produce predictable ("random-looking" but not really random) output depending on the implementation. The resulting string is not suitable when you need to guarantee uniqueness or unpredictability.
Even if it produced 6 uniformly random, unpredictable characters, you can expect to see a duplicate after generating only about 50,000 strings, due to the birthday paradox. (sqrt(36^6) = 46656)
Math.random is bad for this kind of thing
server side
Use node crypto module -
var crypto = require("crypto");
var id = crypto.randomBytes(20).toString('hex');
// "bb5dc8842ca31d4603d6aa11448d1654"
The resulting string will be twice as long as the random bytes you generate; each byte encoded to hex is 2 characters. 20 bytes will be 40 characters of hex.
client side
Use the browser's crypto module, crypto.getRandomValues -
The crypto.getRandomValues() method lets you get cryptographically strong random values. The array given as the parameter is filled with random numbers (random in its cryptographic meaning).
// dec2hex :: Integer -> String
// i.e. 0-255 -> '00'-'ff'
function dec2hex (dec) {
return dec.toString(16).padStart(2, "0")
}
// generateId :: Integer -> String
function generateId (len) {
var arr = new Uint8Array((len || 40) / 2)
window.crypto.getRandomValues(arr)
return Array.from(arr, dec2hex).join('')
}
console.log(generateId())
// "82defcf324571e70b0521d79cce2bf3fffccd69"
console.log(generateId(20))
// "c1a050a4cd1556948d41"
A step-by-step console example -
> var arr = new Uint8Array(4) # make array of 4 bytes (values 0-255)
> arr
Uint8Array(4) [ 0, 0, 0, 0 ]
> window.crypto
Crypto { subtle: SubtleCrypto }
> window.crypto.getRandomValues()
TypeError: Crypto.getRandomValues requires at least 1 argument, but only 0 were passed
> window.crypto.getRandomValues(arr)
Uint8Array(4) [ 235, 229, 94, 228 ]
For IE11 support you can use -
(window.crypto || window.msCrypto).getRandomValues(arr)
For browser coverage see https://caniuse.com/#feat=getrandomvalues
client side (old browsers)
If you must support old browsers, consider something like uuid -
const uuid = require("uuid");
const id = uuid.v4();
// "110ec58a-a0f2-4ac4-8393-c866d813b8d1"
Short, easy and reliable
Returns exactly 5 random characters, as opposed to some of the top rated answers found here.
Math.random().toString(36).slice(2, 7);
Here's an improvement on doubletap's excellent answer. The original has two drawbacks which are addressed here:
First, as others have mentioned, it has a small probability of producing short strings or even an empty string (if the random number is 0), which may break your application. Here is a solution:
(Math.random().toString(36)+'00000000000000000').slice(2, N+2)
Second, both the original and the solution above limit the string size N to 16 characters. The following will return a string of size N for any N (but note that using N > 16 will not increase the randomness or decrease the probability of collisions):
Array(N+1).join((Math.random().toString(36)+'00000000000000000').slice(2, 18)).slice(0, N)
Explanation:
Pick a random number in the range [0,1), i.e. between 0 (inclusive) and 1 (exclusive).
Convert the number to a base-36 string, i.e. using characters 0-9 and a-z.
Pad with zeros (solves the first issue).
Slice off the leading '0.' prefix and extra padding zeros.
Repeat the string enough times to have at least N characters in it (by Joining empty strings with the shorter random string used as the delimiter).
Slice exactly N characters from the string.
Further thoughts:
This solution does not use uppercase letters, but in almost all cases (no pun intended) it does not matter.
The maximum string length at N = 16 in the original answer is measured in Chrome. In Firefox it's N = 11. But as explained, the second solution is about supporting any requested string length, not about adding randomness, so it doesn't make much of a difference.
All returned strings have an equal probability of being returned, at least as far as the results returned by Math.random() are evenly distributed (this is not cryptographic-strength randomness, in any case).
Not all possible strings of size N may be returned. In the second solution this is obvious (since the smaller string is simply being duplicated), but also in the original answer this is true since in the conversion to base-36 the last few bits may not be part of the original random bits. Specifically, if you look at the result of Math.random().toString(36), you'll notice the last character is not evenly distributed. Again, in almost all cases it does not matter, but we slice the final string from the beginning rather than the end of the random string so that short strings (e.g. N=1) aren't affected.
Update:
Here are a couple other functional-style one-liners I came up with. They differ from the solution above in that:
They use an explicit arbitrary alphabet (more generic, and suitable to the original question which asked for both uppercase and lowercase letters).
All strings of length N have an equal probability of being returned (i.e. strings contain no repetitions).
They are based on a map function, rather than the toString(36) trick, which makes them more straightforward and easy to understand.
So, say your alphabet of choice is
var s = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
Then these two are equivalent to each other, so you can pick whichever is more intuitive to you:
Array(N).join().split(',').map(function() { return s.charAt(Math.floor(Math.random() * s.length)); }).join('');
and
Array.apply(null, Array(N)).map(function() { return s.charAt(Math.floor(Math.random() * s.length)); }).join('');
Edit:
I seems like qubyte and Martijn de Milliano came up with solutions similar to the latter (kudos!), which I somehow missed. Since they don't look as short at a glance, I'll leave it here anyway in case someone really wants a one-liner :-)
Also, replaced 'new Array' with 'Array' in all solutions to shave off a few more bytes.
The most compact solution, because slice is shorter than substring. Subtracting from the end of the string allows to avoid floating point symbol generated by the random function:
Math.random().toString(36).slice(-5);
or even
(+new Date).toString(36).slice(-5);
Update: Added one more approach using btoa method:
btoa(Math.random()).slice(0, 5);
btoa(+new Date).slice(-7, -2);
btoa(+new Date).substr(-7, 5);
// Using Math.random and Base 36:
console.log(Math.random().toString(36).slice(-5));
// Using new Date and Base 36:
console.log((+new Date).toString(36).slice(-5));
// Using Math.random and Base 64 (btoa):
console.log(btoa(Math.random()).slice(0, 5));
// Using new Date and Base 64 (btoa):
console.log(btoa(+new Date).slice(-7, -2));
console.log(btoa(+new Date).substr(-7, 5));
A newer version with es6 spread operator:
[...Array(30)].map(() => Math.random().toString(36)[2]).join('')
The 30 is an arbitrary number, you can pick any token length you want
The 36 is the maximum radix number you can pass to numeric.toString(), which means all numbers and a-z lowercase letters
The 2 is used to pick the 3rd index from the random string which looks like this: "0.mfbiohx64i", we could take any index after 0.
Something like this should work
function randomString(len, charSet) {
charSet = charSet || 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
var randomString = '';
for (var i = 0; i < len; i++) {
var randomPoz = Math.floor(Math.random() * charSet.length);
randomString += charSet.substring(randomPoz,randomPoz+1);
}
return randomString;
}
Call with default charset [a-zA-Z0-9] or send in your own:
var randomValue = randomString(5);
var randomValue = randomString(5, 'PICKCHARSFROMTHISSET');
function randomstring(L) {
var s = '';
var randomchar = function() {
var n = Math.floor(Math.random() * 62);
if (n < 10) return n; //1-10
if (n < 36) return String.fromCharCode(n + 55); //A-Z
return String.fromCharCode(n + 61); //a-z
}
while (s.length < L) s += randomchar();
return s;
}
console.log(randomstring(5));
Random String Generator (Alpha-Numeric | Alpha | Numeric)
/**
* Pseudo-random string generator
* http://stackoverflow.com/a/27872144/383904
* Default: return a random alpha-numeric string
*
* #param {Integer} len Desired length
* #param {String} an Optional (alphanumeric), "a" (alpha), "n" (numeric)
* #return {String}
*/
function randomString(len, an) {
an = an && an.toLowerCase();
var str = "",
i = 0,
min = an == "a" ? 10 : 0,
max = an == "n" ? 10 : 62;
for (; i++ < len;) {
var r = Math.random() * (max - min) + min << 0;
str += String.fromCharCode(r += r > 9 ? r < 36 ? 55 : 61 : 48);
}
return str;
}
console.log(randomString(10)); // i.e: "4Z8iNQag9v"
console.log(randomString(10, "a")); // i.e: "aUkZuHNcWw"
console.log(randomString(10, "n")); // i.e: "9055739230"
While the above uses additional checks for the desired A/N, A, N output,
let's break it down the to the essentials (Alpha-Numeric only) for a better understanding:
Create a function that accepts an argument (desired length of the random String result)
Create an empty string like var str = ""; to concatenate random characters
Inside a loop create a rand index number from 0 to 61 (0..9+A..Z+a..z = 62)
Create a conditional logic to Adjust/fix rand (since it's 0..61) incrementing it by some number (see examples below) to get back the right CharCode number and the related Character.
Inside the loop concatenate to str a String.fromCharCode( incremented rand )
Let's picture the ASCII Character table ranges:
_____0....9______A..........Z______a..........z___________ Character
| 10 | | 26 | | 26 | Tot = 62 characters
48....57 65..........90 97..........122 CharCode ranges
Math.floor( Math.random * 62 ) gives a range from 0..61 (what we need).
Let's fix the random to get the correct charCode ranges:
| rand | charCode | (0..61)rand += fix = charCode ranges |
------+----------+----------+--------------------------------+-----------------+
0..9 | 0..9 | 48..57 | rand += 48 = 48..57 |
A..Z | 10..35 | 65..90 | rand += 55 /* 90-35 = 55 */ = 65..90 |
a..z | 36..61 | 97..122 | rand += 61 /* 122-61 = 61 */ = 97..122 |
The conditional operation logic from the table above:
rand += rand>9 ? ( rand<36 ? 55 : 61 ) : 48 ;
// rand += true ? ( true ? 55 else 61 ) else 48 ;
From the explanation above, here's the resulting alpha-numeric snippet:
function randomString(len) {
var str = ""; // String result
for (var i = 0; i < len; i++) { // Loop `len` times
var rand = Math.floor(Math.random() * 62); // random: 0..61
var charCode = rand += rand > 9 ? (rand < 36 ? 55 : 61) : 48; // Get correct charCode
str += String.fromCharCode(charCode); // add Character to str
}
return str; // After all loops are done, return the concatenated string
}
console.log(randomString(10)); // i.e: "7GL9F0ne6t"
Or if you will:
const randomString = (n, r='') => {
while (n--) r += String.fromCharCode((r=Math.random()*62|0, r+=r>9?(r<36?55:61):48));
return r;
};
console.log(randomString(10))
To meet requirement [a-zA-Z0-9] and length of 5 characters, use
For Browser:
btoa(Math.random().toString()).substring(10,15);
For NodeJS:
Buffer.from(Math.random().toString()).toString("base64").substring(10,15);
Lowercase letters, uppercase letters, and numbers will occur.
(it's typescript compatible)
The simplest way is:
(new Date%9e6).toString(36)
This generate random strings of 5 characters based on the current time. Example output is 4mtxj or 4mv90 or 4mwp1
The problem with this is that if you call it two times on the same second, it will generate the same string.
The safer way is:
(0|Math.random()*9e6).toString(36)
This will generate a random string of 4 or 5 characters, always diferent. Example output is like 30jzm or 1r591 or 4su1a
In both ways the first part generate a random number. The .toString(36) part cast the number to a base36 (alphadecimal) representation of it.
Here are some easy one liners. Change new Array(5) to set the length.
Including 0-9a-z
new Array(5).join().replace(/(.|$)/g, function(){return ((Math.random()*36)|0).toString(36);})
Including 0-9a-zA-Z
new Array(5).join().replace(/(.|$)/g, function(){return ((Math.random()*36)|0).toString(36)[Math.random()<.5?"toString":"toUpperCase"]();});
Codegolfed for ES6 (0-9a-z)
Array(5).fill().map(n=>(Math.random()*36|0).toString(36)).join('')
I know everyone has got it right already, but i felt like having a go at this one in the most lightweight way possible(light on code, not CPU):
function rand(length, current) {
current = current ? current : '';
return length ? rand(--length, "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz".charAt(Math.floor(Math.random() * 60)) + current) : current;
}
console.log(rand(5));
It takes a bit of time to wrap your head around, but I think it really shows how awesome javascript's syntax is.
Generate a secure random alphanumeric Base-62 string:
function generateUID(length)
{
return window.btoa(String.fromCharCode(...window.crypto.getRandomValues(new Uint8Array(length * 2)))).replace(/[+/]/g, "").substring(0, length);
}
console.log(generateUID(22)); // "yFg3Upv2cE9cKOXd7hHwWp"
console.log(generateUID(5)); // "YQGzP"
There is no best way to do this. You can do it any way you prefer, as long as the result suits your requirements. To illustrate, I've created many different examples, all which should provide the same end-result
Most other answers on this page ignore the upper-case character requirement.
Here is my fastest solution and most readable. It basically does the same as the accepted solution, except it is a bit faster.
function readableRandomStringMaker(length) {
for (var s=''; s.length < length; s += 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'.charAt(Math.random()*62|0));
return s;
}
console.log(readableRandomStringMaker(length));
// e3cbN
Here is a compact, recursive version which is much less readable:
const compactRandomStringMaker = (length) => length-- && "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0) + (compactRandomStringMaker(length)||"");
console.log(compactRandomStringMaker(5));
// DVudj
A more compact one-liner:
Array(5).fill().map(()=>"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62)).join("")
// 12oEZ
A variation of the above:
" ".replaceAll(" ",()=>"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62))
The most compact one-liner, but inefficient and unreadable - it adds random characters and removes illegal characters until length is l:
((l,f=(p='')=>p.length<l?f(p+String.fromCharCode(Math.random()*123).replace(/[^a-z0-9]/i,'')):p)=>f())(5)
A cryptographically secure version, which is wasting entropy for compactness, and is a waste regardless because the generated string is so short:
[...crypto.getRandomValues(new Uint8Array(999))].map((c)=>String.fromCharCode(c).replace(/[^a-z0-9]/i,'')).join("").substr(0,5)
// 8fzPq
Or, without the length-argument it is even shorter:
((f=(p='')=>p.length<5?f(p+String.fromCharCode(Math.random()*123).replace(/[^a-z0-9]/i,'')):p)=>f())()
// EV6c9
Then a bit more challenging - using a nameless recursive arrow function:
((l,s=((l)=>l--&&"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0)+(s(l)||""))) => s(l))(5);
// qzal4
This is a "magic" variable which provides a random character every time you access it:
const c = new class { [Symbol.toPrimitive]() { return "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0) } };
console.log(c+c+c+c+c);
// AgMnz
A simpler variant of the above:
const c=()=>"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".charAt(Math.random()*62|0);
c()+c()+c()+c()+c();
// 6Qadw
In case anyone is interested in a one-liner (although not formatted as such for your convenience) that allocates the memory at once (but note that for small strings it really does not matter) here is how to do it:
Array.apply(0, Array(5)).map(function() {
return (function(charset){
return charset.charAt(Math.floor(Math.random() * charset.length))
}('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'));
}).join('')
You can replace 5 by the length of the string you want. Thanks to #AriyaHidayat in this post for the solution to the map function not working on the sparse array created by Array(5).
If you are using Lodash or Underscore, then it so simple:
var randomVal = _.sample('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', 5).join('');
const c = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
const s = [...Array(5)].map(_ => c[~~(Math.random()*c.length)]).join('')
Here's the method I created.
It will create a string containing both uppercase and lowercase characters.
In addition I've included the function that will created an alphanumeric string too.
Working examples:
http://jsfiddle.net/greatbigmassive/vhsxs/ (alpha only)
http://jsfiddle.net/greatbigmassive/PJwg8/ (alphanumeric)
function randString(x){
var s = "";
while(s.length<x&&x>0){
var r = Math.random();
s+= String.fromCharCode(Math.floor(r*26) + (r>0.5?97:65));
}
return s;
}
Upgrade July 2015
This does the same thing but makes more sense and includes all letters.
var s = "";
while(s.length<x&&x>0){
v = Math.random()<0.5?32:0;
s += String.fromCharCode(Math.round(Math.random()*((122-v)-(97-v))+(97-v)));
}
One liner:
Array(15).fill(null).map(() => Math.random().toString(36).substr(2)).join('')
// Outputs: 0h61cbpw96y83qtnunwme5lxk1i70a6o5r5lckfcyh1dl9fffydcfxddd69ada9tu9jvqdx864xj1ul3wtfztmh2oz2vs3mv6ej0fe58ho1cftkjcuyl2lfkmxlwua83ibotxqc4guyuvrvtf60naob26t6swzpil
Improved #Andrew's answer above :
Array.from({ length : 1 }, () => Math.random().toString(36)[2]).join('');
Base 36 conversion of the random number is inconsistent, so selecting a single indice fixes that. You can change the length for a string with the exact length desired.
Assuming you use underscorejs it's possible to elegantly generate random string in just two lines:
var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
var random = _.sample(possible, 5).join('');
function randomString (strLength, charSet) {
var result = [];
strLength = strLength || 5;
charSet = charSet || 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
while (strLength--) { // (note, fixed typo)
result.push(charSet.charAt(Math.floor(Math.random() * charSet.length)));
}
return result.join('');
}
This is as clean as it will get. It is fast too, http://jsperf.com/ay-random-string.
Fast and improved algorithm. Does not guarantee uniform (see comments).
function getRandomId(length) {
if (!length) {
return '';
}
const possible =
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
let array;
if ('Uint8Array' in self && 'crypto' in self && length <= 65536) {
array = new Uint8Array(length);
self.crypto.getRandomValues(array);
} else {
array = new Array(length);
for (let i = 0; i < length; i++) {
array[i] = Math.floor(Math.random() * 62);
}
}
let result = '';
for (let i = 0; i < length; i++) {
result += possible.charAt(array[i] % 62);
}
return result;
}
How about this compact little trick?
var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
var stringLength = 5;
function pickRandom() {
return possible[Math.floor(Math.random() * possible.length)];
}
var randomString = Array.apply(null, Array(stringLength)).map(pickRandom).join('');
You need the Array.apply there to trick the empty array into being an array of undefineds.
If you're coding for ES2015, then building the array is a little simpler:
var randomString = Array.from({ length: stringLength }, pickRandom).join('');
You can loop through an array of items and recursively add them to a string variable, for instance if you wanted a random DNA sequence:
function randomDNA(len) {
len = len || 100
var nuc = new Array("A", "T", "C", "G")
var i = 0
var n = 0
s = ''
while (i <= len - 1) {
n = Math.floor(Math.random() * 4)
s += nuc[n]
i++
}
return s
}
console.log(randomDNA(5));
Case Insensitive Alphanumeric Chars:
function randStr(len) {
let s = '';
while (s.length < len) s += Math.random().toString(36).substr(2, len - s.length);
return s;
}
// usage
console.log(randStr(50));
The benefit of this function is that you can get different length random string and it ensures the length of the string.
Case Sensitive All Chars:
function randStr(len) {
let s = '';
while (len--) s += String.fromCodePoint(Math.floor(Math.random() * (126 - 33) + 33));
return s;
}
// usage
console.log(randStr(50));
Custom Chars
function randStr(len, chars='abc123') {
let s = '';
while (len--) s += chars[Math.floor(Math.random() * chars.length)];
return s;
}
// usage
console.log(randStr(50));
console.log(randStr(50, 'abc'));
console.log(randStr(50, 'aab')); // more a than b
The problem with responses to "I need random strings" questions (in whatever language) is practically every solution uses a flawed primary specification of string length. The questions themselves rarely reveal why the random strings are needed, but I would challenge you rarely need random strings of length, say 8. What you invariably need is some number of unique strings, for example, to use as identifiers for some purpose.
There are two leading ways to get strictly unique strings: deterministically (which is not random) and store/compare (which is onerous). What do we do? We give up the ghost. We go with probabilistic uniqueness instead. That is, we accept that there is some (however small) risk that our strings won't be unique. This is where understanding collision probability and entropy are helpful.
So I'll rephrase the invariable need as needing some number of strings with a small risk of repeat. As a concrete example, let's say you want to generate a potential of 5 million IDs. You don't want to store and compare each new string, and you want them to be random, so you accept some risk of repeat. As example, let's say a risk of less than 1 in a trillion chance of repeat. So what length of string do you need? Well, that question is underspecified as it depends on the characters used. But more importantly, it's misguided. What you need is a specification of the entropy of the strings, not their length. Entropy can be directly related to the probability of a repeat in some number of strings. String length can't.
And this is where a library like EntropyString can help. To generate random IDs that have less than 1 in a trillion chance of repeat in 5 million strings using entropy-string:
import {Random, Entropy} from 'entropy-string'
const random = new Random()
const bits = Entropy.bits(5e6, 1e12)
const string = random.string(bits)
"44hTNghjNHGGRHqH9"
entropy-string uses a character set with 32 characters by default. There are other predefined characters sets, and you can specify your own characters as well. For example, generating IDs with the same entropy as above but using hex characters:
import {Random, Entropy, charSet16} from './entropy-string'
const random = new Random(charSet16)
const bits = Entropy.bits(5e6, 1e12)
const string = random.string(bits)
"27b33372ade513715481f"
Note the difference in string length due to the difference in total number of characters in the character set used. The risk of repeat in the specified number of potential strings is the same. The string lengths are not. And best of all, the risk of repeat and the potential number of strings is explicit. No more guessing with string length.
One-liner using map that gives you full control on the length and characters.
const rnd = (len, chars='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789') => [...Array(len)].map(() => chars.charAt(Math.floor(Math.random() * chars.length))).join('')
console.log(rnd(12))

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